PocketBackup (
www.spritesoftware.com) will backup the contents of your SD Card, but I don't know if it will sync the contents.
I have a large SD Card that most of my programs run off of. Additionally, I do my Axim backups to the SD Card (with the built in backup APP). Once a week, I pop out the SD card, stick it in my PC card reader, and copy the contents to my hard drive.
This system has worked perfectly for me, and has saved my bacon on hard resets.